The vast majority of any vehicle’s environmental impact will be created by the fuel it burns and the emissions that come out of its tailpipe.

“The best way to go about a vehicle purchase is to figure out the ‘most’ vehicle you really need, whether that is a pickup, a minivan, or a compact car, and then look for the most efficient version of that vehicle,” says Don Anair, a senior analyst with the Union of Concerned Scientists’ (UCS) Clean Vehicles Program.

Once you’ve got that figured out, consider the following:

Fuel Efficiency: Extracting, refining, and transporting fossil fuels is energy-intensive and can strain local ecosystems. When motor vehicles burn those fuels they produce lots of greenhouse gases, about a quarter of the United States’ total carbon dioxide (CO2) emissions. And they seriously pollute the air we breathe with smog and toxins. Using less fuel for each mile you drive is key to going green—so make high mileage a priority.

Emissions: The federal government has minimum emissions standards. The U.S. Environmental Protection Agency (EPA) gives cars and trucks scores for both greenhouse gas emissions, which drive climate change, and hazardous air pollution.

Check out EPA’s Green Vehicle Guide. It rates and compares makes and models on the all-important category of fuel efficiency, but you can also see how the different models stack up when it comes to greenhouse gas emissions and air pollution, two critical components in making your car choice as green as it can be.

Fuel Types: Eco-savvy car shoppers have several fuel choices to ponder—but still no perfectly green choice.

Gasoline and Diesel: New “clean” diesel engines and their high-efficiency gasoline counterparts are probably a toss-up environmentally, experts say, because each has its pros and cons. Although diesels traditionally have been heavy polluters, some new models are on the American Council for an Energy-Efficient Economy’s (ACEEE) list of the top green models.

Diesels burn less fuel and run up to 40 percent farther on a single gallon. That gallon of diesel does produce more greenhouse gases than its gasoline counterpart, but because it can move the vehicle significantly farther, diesel’s greenhouse emissions per mile are typically lower than gasoline. Despite recent improvements, however, diesels still produce more soot- and smog-forming nitrogen oxide emissions, which may play their own role in warming the planet and also have consequences for human health.

There has been much talk about the potentialof biodiesel, the renewable fuel made from plant oils (such as soybean or palm), animal fats, or even repurposed cooking oil, that can be used to fuel diesel engines. But questions abound concerning just how green this biological fuel source really is. Producing the product’s base crops can cause major shifts in land use, including deforestation, and could actually have a negative impact in the fight against climate change. Unless you use recycled cooking oils or have a local supplier, knowing the source of your biodiesel can be difficult.

Hybrids: Most hybrids pair a gasoline engine with an electric motor and battery capable of powering the car solo during times of low demand. When the engine runs, it recharges the battery—so the vehicle is never “plugged in.” These efficient systems can cut fuel consumption by 30 to 50 percent over the gasoline-fueled versions of the same auto models. They can also reduce smog pollution drastically—as much as 90 percent over even the cleanest gasoline-only vehicles.

But UCS’s Don Anair cautions that not all hybrids are as green as they seem. “When we looked at the models available [we found] automakers are kind of mixed on how well they are implementing the technology,” he says. “Hybrid technology allows 30 to 50 percent efficiency gains. Some automakers are implementing that technology to achieve the cleanest smog-forming emissions and highest levels of greenhouse gas reductions. But others are not.”

To help consumers compare hybrid efficiency among models, and see which choices are really green, UCS has created the Hybrid Scorecard.

Electric Vehicles: The first mainstream electric vehicles are due to hit U.S. markets later this year, allowing owners to avoid gas stations entirely or use them only to prime small backup engines while driving largely under pure electric power. These cars promise staggering mpg ratings: The Chevy Volt, with its small gasoline backup, promises “infinite” mpg under electric power only to 230 mpg in city driving conditions. That’s because gas use kicks in after 40 miles, and three-fourths of drivers’ trips never reach that distance, according to Tony Posawatz, the vehicle line director for the Volt.

EPA is developing new fuel economy labels for electric cars.

A car that runs on an electric battery burns no fuel. But that doesn’t mean that no fuel was burned to produce the energy that powers the battery. “You have zero tailpipe emissions when running on electric only,” UCS’s Anair says. “But there are upstream emissions associated with the electricity you’re using to charge the battery.”

Currently power plants primarily use fossil fuels such as coal to generate the energy that will power electric cars, so there is really no such thing as a free ride. Still, the cars do represent a significant environmental improvement.

Anair believes the total environmental impact of running an electric vehicle off the grid is much less than a gasoline engine, and probably about the same or a bit better than a very good hybrid. But he sees a potential for far greater green savings.

“In the future, to really get the benefits of electric vehicle technology, we have to clean up the electrical grid by going to renewable and zero-emission electricity,” he says.